FIGS. 4a and 4b illustrate prior art phase shifters. FIG. 4a indicates a passive type phase shifter consisting of resistances R and R/2, a capacitance C and an inductance L while FIG. 4b indicates an active type phase shifter consisting of an operational amplifier AMP, resistances R and R' and a capacitance C.
In the phase shifter indicated in FIG. 4a, if the inductance L and the capacitance C are selected so as to satisfy: ##EQU1## the frequency characteristics thereof are represented by curves as indicated in FIG. 5. As it can be clearly seen from the figure, the amplitude is constant and the phase is given by: ##EQU2##
Also in the phase shifter indicated in FIG. 4b, in the same way, the frequency characteristics that the amplitude is constant and the phase is given by Equation 3 are obtained by selecting the resistance R and the capacitance C so as to satisfy: ##EQU3##
However, in the case when the phase is controlled in a wide range (at least from 0.degree. to 360.degree.), the phase shifters indicated in FIGS. 4a and 4b are not suitable because it is necessary to connect at least 2 of the circuits indicated in FIGS. 4a and 4b in cascade as it is clearly seen from FIG. 5 and both the values L and C should be variable for the circuit indicated in FIG. 4a as it can be understood from Equations (1) and (2). Further, in order to keep the amplitude constant, the relation between L and C should be kept constant and it is difficult to control the phase from the exterior. Next, it makes the phase shifter indicated in FIG. 4b unsuitable for use in a high frequency circuit that an operational amplifier AMP is required therefor. That is, it is not easy to realize a high input impedance, a low output impedance, a high gain, etc. which are necessary conditions for the operational amplifier in a high frequency. In prior art, phase shifters other than those indicated in FIGS. 4a and 4b in order to keep the amplitude constant both the inductance and the capacitance should varied and sometimes the amplitude can not be kept constant. Thus it is unsuitable for the phase control from the exterior and in particular for automatic control.